1. Field of the Invention
The invention relates in general to a data processing method for a holographic data storage system, and more particularly to a data processing method adopting soft decision in demodulation.
2. Description of the Related Art
Compared to the storage technique of magnetic discs and DVD in the present market, a holographic data storage system is a new trend, which has higher capacity and transmission rate.
Referring to FIG. 1, a schematic diagram of a conventional holographic data storage system is shown. Normally, the holographic data storage system 100 is composed of a signal beam 12, data plane 14, reference beam 16, storage medium 18, data beam 22 and photo detecting apparatus 20.
In the holographic data storage system 100, a monochromatic light source, such as a laser source, is split into two beams through a splitter (not shown). One of the two beams arrives and passes the data plane 14 to form the signal beam 12 carrying image information. The other beam is used as the reference beam 16.
When the signal beam 12 and the reference beam 16 are simultaneously focused on the storage medium 18, interference fringes are formed by the signal beam 12 and the reference beam 16 on a focal point 24, which can be regarded as a grating formed on the focal point 24 of the storage medium 24. This operation is equivalent to completion of data writing.
Following that, when only the reference beam 16 arrives the storage medium 18, the data beam 22 is outputted in an original extension direction of the signal beam 12 (i.e. an emergence angle of the signal beam 12). Then, the photo detecting apparatus 20 is placed in the progressing direction of the data beam 22 to catch the image information originally located on the data plane 14. This operation is equivalent to completion of data reading.
Besides, by using a multiplexing technique, such as angle multiplexing or frequency multiplexing, the reference beams with different incident angles or frequencies can also be repeatedly recorded on the same focal point.
Generally speaking, the data plane 14 is a so-called spatial light modulator (SLM), such as a digital micro-mirror device (DMD) or liquid crystal display (LCD). Each the DMD and the LCD includes a number of display pixels arrayed in a matrix form. Each pixel on the matrix can be controlled by electric signals to be transparent or opaque according to the to-be-stored data. The transparent pixel is normally regarded as an “on” channel bit or a binary bit “1”, while the opaque pixel is an “off” channel bit or a binary bit “0”. That is to say, the image data of the data plane 14 is a combination of all possible states of the display pixels to be transparent or opaque. For this reason, the signal beam 12 passing the SLM carries the image information.
Furthermore, the storage medium 18 is photopolymer, and the photo detecting apparatus 20 is a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Similarly, each of the CCD and the CMOS includes a number of photo sensing units arrayed in a matrix form for receiving the image information carried by the data beam 22.
However, due to optical characteristics, the opaque pixels will be interfered by the transparent pixels in the image information. That is to say, if the number of the transparent pixels on the data plane 14 is larger than that of the opaque pixels, when data writing, the opaque pixels in the image information will be over-interfered and result in the lower signal-to-noise ratio (SNR). As a result, when data reading, the photo detecting apparatus 20 will receive incorrect image information and produce data reading errors.
Therefore, the conventional holographic data storage system needs to modulate the received raw data, that is, to add channel bits into the raw data, for reducing the number of the transparent pixels in the image information. In this way, the opaque pixels can be prevented from being over-interfered by the transparent pixels and the SNR can be effectively improved. The demodulation process in the data reading operation will become smoother and the error rate of data reading can thus be reduced.
The U.S. Pat. No. 6,549,664 discloses a data processing method. Although this data processing method improves the issues of too many transparent pixels by using an encoding lookup table, quite a few data reading errors are still be generated owing to adopting hard decision in the reading process.
Based on the hard decision, the analog signal with data and noise mixed together is directly determined to be a digital signal after the photo detecting apparatus 20 receives optical signals, which causes partial data to be removed together with the noise and thus increases the error rate of data reading.